Increasing kraft pulp yield with hydroxylamine



United States Patent O 3,401,076 I INCREASING KRAFT PULP YIELD WITH 'HYDROXYLAMINE Richard M. L. Paterson, Camillus, N.Y., assignor to Allied Chemical Corporation, New York, N.Y., a corporation of New York No Drawing. Filed Aug. 12, 1965, Ser. No. 479,308 6 Claims. (Cl. 16270) This invention relates to the production of cellulose pulp by the so-called kraft or sulfate process and more particularly, to the addition of hydroxylamine and/or salts of hydroxylamine to the kraft pulping liquor to increase the pulp yield and provide an improved kraft p p- In the kraft (or sulfate) process, wood chips are digested with an alkaline aqueous liquor such as sodium hydroxide and sulfide to remove the organic contaminants from the wood of which lignin is the chief component. In a typical process, the chips are digested in a vessel in about a solution of sodium hydroxide which contains about 25 mole percent of sodium sulfide together with a certain amount of sodium carbonate. The reaction is usually conducted at temperatures of about 150-180 C. for a period of time ranging from 1 to 4 hours. The waste liquor and pulp washings containing the resultant organic residues termed black liquor are thereafter concentrated and burned to recover the pulping chemicals. The smelt from the recovery furnace is dissolved (green liquor), causticized with milkof lime and the resultant white liquor is returned to the digester. Normally, sodium sulfate (salt cake) is added to the black liquor to make up the sodium and sulfur losses from the process.

A disadvantage of the process is that the combination of the high alkalinity and elevated temperatures and pressures employed in the digestion cause certain deleterious effects on the product yield. More specifically, under the severe conditions of the process, the cellulose chains are to some extent hydrolyzed or broken, resulting in decreased yields of the desired product.

In View of the extremely large tonnage of cellulosic material processed eachyear, the art has resorted to a variety of procedures in order to increase the yield of product while still maintaining product quality. It should be remembered that even a slight increase in yield over the yields obtained by present commercial processes would be most beneficial in terms of cost. Moreover, it would be highly desirable to increase the yield while still maintaining or, more preferably, improving the quality of the pulp. Previous attempts directed toward this purpose resulted in the discovery of a variety of additives including sodium borohydride, polysulfides and various wetting agents which when added to the paper pulping digestion process improved the yields and pulp characteristics. However, the beneficial results obtained are usually uneconomical or accompanied by the sacrifice of one or more other characteristics of the process such as decrease in brightness of a bleachable pulp and decreased viscosities. The art is, therefore, still in need of an additive which is inexpensive with relation to the amount of savings from increased yields of pulp, and which produces a pulp having increased viscosities without loss of pulp brightness.

An object of the present invention is to provide an improved method for producing cellulose pulp from the kraft process.

Another object is to obtain increased yields of pulp products by the kraft process.

Another object is to provide an improved method for obtaining pulp from the kraft process having increased viscosities.

Other objects and advantages will be apparent from the following description of the invention.

In the course of our experimentation, we discovered that if hydroxylamine or a salt of hydroxylamine is incorporated in the kraft pulping liquor in very small amounts that the yield of pulp was increased and also the viscosity of the pulp was increased. While it is not completely understood why the presence of hydroxylamine or one of its salts in the digester liquor results in an increase in the viscosity and the yield of the pulp produced, it is believed that the hydroxylamine increases the yield by blocking the carbonyl and hemiacetyl end groups responsible for alkaline degradation of cellulose and may function also as a free radical scavenger to promote the pulping reaction. Whatever the theory or phenomena involved, it has been discovered that improved yields and increased viscosity of pulp from the kraft process can be obtained by incorporating in the digester liquor, hydroxylamine or one of its salts or mixtures thereof in an amount of from 0.1 to 10.0% by weight, based on the weight of the oven dry Wood.

The additive to the pulp digester may be hydroxylamine or one of its salts such as hydroxylamine sulfite, hydroxylamine sulfate, hydroxylamine bisulfite, preferably hydroxylamine bi-sulfite and hydroxylamine sulfate. If desired, the additive may be a compound or mixture of compounds which are normally inert or substantially inert, but when incorporated in the pulping liquor will form hydroxylamine in situ in an amount effective for the process. Examples of compounds which form hydroxylamine in the pulping liquor under the conditions of operations include hydroxylamine sulfite, hydroxylamine sulfate, hydroxylamine bisulfite, hydroxylamine chloride, and acetoxime. The additive of the present invention added to the pulping liquor may be varied over a range of about 0.1 to 10% by weight based on the weight of the oven dry wood. About 1-5% is preferred because when employing this amount, excellent results are obtained and is, therefore, desirable for economic reasons. When a hydroxylamine salt or a compound or mixture of compounds capable of forming hydroxylamine in a pulping liquor is employed, an amount of alkali (sodium hydroxide, soda ash or white liquor) equivalent to the additive is added to the digester in addition to the normal charge of white or white and black liquor recycled to the digester. The manner in which the additive is added to pulping liquor is not critical and the addition may be performed by any of the convenient procedures now known in the art. For example, the additive which may be used in pure *form or alternatively as a solution, can be added directly to the digester with the other pulping liquors. In another technique, the additive may be sprayed on the wood chips prior to entering the digester. Another convenient procedure is to dissolve the additive in the white liquor recycled from the process and add the white liquor containing the dissolved additive to the digester in contact with the other pulping chemicals.

The yields of pulp produced by the employment of the additive of the present invention are generally in the range of 48% to 58% and occasionally as high as 65% based on the original amount of wood chips or about 1 to 10% higher than that obtained with a conventional kraft liquor. The method of this invention is particularly useful with coniferous woods, especially pines, since these present a much greater problem in pulping to obtain increased yield of a product of desirable properties.

The conditions employed for the pulping process are generally the same as employed in the conventional kraft or sulfate pulping process.

An advantage of the process of this invention is that the viscosity of the bleachable pulp is increased when the additive of'the present invention is incorporated in the digester liquor. This is indicative that the strength of the pulp is not materially lowered. A measure of the strength of pulp is the Cupriethylenediamine Disperse Viscosity of Pulp, TAPPI standard T230 Sm50. This disperse viscosity is a measure of the chain length of the cellulose molecules present in the fibers, and is an indication of the amount of degradation due, initially, to the pulping operation and later to the bleaching operations. The superior viscosity of the hydroxylamine pulp is reflected in better mullen and fold values before bleaching and partial development of these properties immediately upon bleaching.

The following examples will tend to futher illustrate the invention.

EXAMPLE I In a small scale experiment designed to simulate kraft pulping 100 parts by weight of ground wood were treated with 900 parts of an aqueous solution containing 25.8 parts of sodium hydroxide, 8.2 parts of sodium sulfide and 5.9 parts of sodium carbonate, at 120 C. for 240 minutes, with from O-5 parts of hydroxylamine sulfate added. The treated pulp was washed, dried and weighed to determine the yield.

Parts by weight of hydroxylamine sulfate added: Pulp yield 74.42

EXAMPLE II 100 parts dry weight of Southern pine wood chips were heated in a 3 ft. digester to 170 C. in 120 minutes with 400 parts of an aqueous solution containing 17 6.35 parts Additive None Hydroxylamine bisuliatc TAPPI permanganate No 21. 6 21. 1 Total yield, percent 48. 4 52. 8 Screened yield, percent 44. 7 48. 1 Viscosity (TAPPI 0.5% Cuene) 35. 4 47.2

As can be seen from the above, addition of hydroxyl- 4 amine as bisulfate increases yield and viscosity ofthe pulp. 1

I claim:

1. An improved method for obtaining wood pulp from the kraft process wherein wood chips are cooked with an alkaline cooking liquor to produce a wood pulp, the improvement which comprises incorporating in the cooking liquor a compound selected from the group consisting of hydroxylamine, hydroxylamine sulfite, hydroxylamirie sulfate and hydroxylamine bisulfite in an amount of 0.1 to 10% by weight based on the dry weight of the cellulosic material.

2. A method according to claim 1 wherein said compound is incorporated in the cooking liquor in an amount of about 15% by weight based on the dry weight of the cellulosic material.

3. An improved method for obtaining cellulosic" pulp from the kraft process wherein a cellulosic material'is cooked with an alkaline cooking liquor to produce a pulp, the improvement which comprises incorporating hydroxylamine in the cooking liquor in an amount of about 15% by weight based on the dry weight of the cellulosic material.

4. An improved method for obtaining cellulosic pulp from the kraft process wherein a cellulosic material is cooked with an alkaline cooking liquor to produce a pulp, the improvement which comprises incorporating hydroxylamine sulfite in the cooking liquor in an amount of about 15% by weight based on the dry weight of the cellulosic material.

5. An improved method for obtaining cellulosic pulp from the kraft process wherein a cellulosic material is cooked with an alkaline cooking liquor to produce a pulp, the improvement which comprises incorporating hydroxylamine sulfate in the cooking liquor in an amount of about 15% by weight based on the dry weight of the cellulosic material.

6. An improved method for obtaining cellulosic pulp from the kraft process wherein a cellulosic material is cooked with an alkaline cooking liquor to produce a pulp, the improvement which comprises incorporating hydroxylamine bisulfite in the cooking liquor in an amount of about l5% by weight based on the dry weight of the cellulosic material.

References Cited UNITED STATES PATENTS 3,161,562 12/1964 Gillaspie 16270 DONALL H. SYLVESTER, Primary Examiner.

H. R. CAINE, Assistant Examiner. 

1. AN IMPROVED METHOD FOR OBTAINING WOOD PULP FROM THE KRAFT PROCESS WHEREIN WOOD CHIPS ARE COOKED WITH AN ALKALINE COOKING LIQUOR TO PRODUCE A WOOD PULP, THE IMPROVEMENT WHICH COMPRISES INCORPORATION IN THE COOKING LIQUOR A COMPOUND SELECTED FROM THE GROUP CONSISTING OF HYDROXYLAMINE, HYDROXYLAMINE SULFITE, HYDROXYLAMINE SULFATE AND HYDROXYLAMINE BISULFITE IN AN AMOUNT OF 0.1 TO 10% BY WEIGHT BASED ON THE DRY WEIGHT OF THE CELLULOSIC MATERIAL. 